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Table of Content

Goal

This is an example of how good (in my opinion) application should look like.

How to launch (TODO)

Either using docker compose, docker or kubernetes.

mvn clean install

Project Structure

Package by feature not by layer.

Great article. Below information is taken from it:

  • Drawbacks of packaging classes by technical concerns:
    • Poor overview of all classes that belong to a feature.
    • Tendency to generic, reused and complex code, which is hard to understand and changes can easily break other use cases as the impact of a change is hard to grasp. Often, this approach leads to central classes containing all methods for every use case. Over time, those methods get more and more abstracted (with extra parameters and generics) to fulfill more use cases.
  • Instead, package by feature and create packages that contain all classes that are required for a feature. The benefits are:
    • Better discoverability and overview
    • Self-contained and independent
    • Simpler code
    • Testability

package-by-layer.PNG

package-by-feature.PNG

Lombok

Do not use Lombok. Records

  • withers should cover almost all cases like DTO's and Request/Response models only exception I see is with entities.

@Entity classes will consist of getters and setters boilerplate, but they make up a very small part of your application, and does not justify adding Lombok. If you're truly bothered by getters/setters you can just make properties public.

Main Arguments

Spring and Hibernate generate bytecode the way the compiler is intending to support it: by generating a java source code file and then compiling it without touching the source file/class the annotation came from. Lombok goes into the parse tree (via non-public APIs) and mutates the AST in place.

  • Reddit comment

<...> while we do code against internal API, they are relatively stable bits. If what lombok does could be done without resorting to internal API, we'd have done something else, but it can't be done, so we resort to internal API usage.

In a few words, the strategy is this: use of JDK internals by libraries is tolerated as long as they notify the user that they’re doing so with add-opens. This is because such libraries become tied to JDK versions, and so must be vigilant when updating, and their users need to be aware of maintenance issues libraries might impose on them (there are also security issues when it comes to libraries that do this at runtime). So, you can do what you like, but if you impose a potential maintenance burden on your users, you must let them know about it. When all loopholes are closed, libraries will need to decide whether to not use internals or acclimate their users to add-opens. In some months there will be no way — using Unsafe, attach, native code etc. — to hack JDK internals without express approval of the application.

More resources:

Lombok and Entity

I've seen the same mistake done many times - @Entity class marked with @Data or @EqualsAndHashCode leads to bad JPA practices:

  • Missing bidirectional synchronized methods.

Whenever a bidirectional association is formed, the application developer must make sure both sides are in-sync at all times. The addPhone() and removePhone() are utility methods that synchronize both ends whenever a child element is added or removed.

Source - Hibernate User Guide

However, we still need to have both sides in sync as otherwise, we break the Domain Model relationship consistency, and the entity state transitions are not guaranteed to work unless both sides are properly synchronized.

Source - Vlad Mihalcea Blog

  • Bad equals and hashcode implementations. Vlad Michalcea Blog

  • Including lazy attributes via @ToString.

Lombok and JPA: What may go wrong?

Lombok and Logger

Lombok solution:

@Slf4j
public class LogExample {
}

//Generates 

public class LogExample {
    private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(LogExample.class);
}

Just use IntelliJ live template and it will generate this for you with a small shortcut. Mine called lger:

private static final org.slf4j.Logger LOGGER = org.slf4j.LoggerFactory.getLogger($CLASS$.class);

Lombok and IntelliJ

Starting from 2020.3 Lombok plugin is now built-in in IntelliJ. Don't forget to disable it, via Settings -> Plugins.

Endpoints

Controller classes are mainly for documenting endpoint, logging requests then delegating to Service Layer and returning responses. They should contain as little business logic as possible.

@GetMapping("{id}")
@ApiOperation("Get information about one employees")
public Employee getEmployee(@PathVariable("id") @Min(1) Long id){
    LOGGER.info("Get employee request. Employee id: {}",id);

    return this.employeeService.getEmployee(id);
}

Don't use ResponseEntity boilerplate:

@GetMapping("{id}")
@ApiOperation("Get information about one employees")
public ResponseEntity<Employee> getEmployee(@PathVariable("id") @Min(1) Long id){
    LOGGER.info("Get employee request. Employee id: {}",id);

    var employee=this.employeeService.getEmployee(id);
    return ResponseEntity.ok(employee);
}

Just return the object:

@GetMapping("{id}")
@ApiOperation("Get information about one employees")
public Employee getEmployee(@PathVariable("id") @Min(1) Long id){
    LOGGER.info("Get employee request. Employee id: {}",id);

    return this.employeeService.getEmployee(id);
}

Unless you have clear need for specific Response, like ResponseEntity.noContent(), ResponseEntity.created(uriLocation).body(body); or ResponseEntity.status(HttpStatus.I_AM_A_TEAPOT).body(body); etc.

Use @ApiOperation to describe an endpoint's purpose. There is no need to describe response or responseContainer as it is inferred by SpringFox automatically.

Define input models.

Request and Response models should be immutable objects and have (this is for easier migration to records):

  • private final instance variables;
  • all-args constructor with @JsonCreator;
  • hashCode, equals, toString;

Use Builder pattern for constructing immutable objects.

After Java 16+ use records.

Whenever you have Collections of objects, don't forget to annotate @NotNull for the container type:

BAD
@NotNull
List<Integer> numbers;

GOOD
@NotNull
List<@NotNull Integer> numbers;

The difference is that in first example, request can be sent with "numbers": [null] and will be a valid request.

https://docs.jboss.org/hibernate/stable/validator/reference/en-US/html_single/#section-object-graph-validation

Don't forget @Validated on @RestController.

Example:

@GetMapping("{id}")
@ApiOperation("Get information about one employees")
public void getEmployee(@PathVariable("id") @Min(1) Long id){<-----------------@Min
    LOGGER.info("Get all employee request. Employee id: {}",id);

    employeeService.getEmployee();
}

If @Validated is missing, then @Min will not be taken into account, thus you could pass -1 as an id.

If you want to have filters for endpoints, like so:

@GetMapping
@ApiOperation("Get information about all employees")
public Set<Employee> getAllEmployees(EmployeeFilter employeeFilter){
    LOGGER.info("Get all employees request. Employee filter {}",employeeFilter);

    return this.employeeService.getAllEmployees(employeeFilter);
}

Make sure to use @JsonCreator and getters, because Spring does not support Records for request params yet as of Spring Boot 2.3. Have not tested with higher versions.

HTTP Methods

GET, HEAD, OPTIONS, and TRACE methods are defined to be safe. Request methods are considered "safe" if their defined semantics are essentially read-only.

Source

Idempotent Methods

PUT, DELETE, and safe request methods are idempotent.

Idempotent - operation can be repeated and the outcome will be the same (the server's state will remain the same). For example, PUT employee will either create or update existing. Doesn't matter, how many times you will repeat the same operation it will always return same response.

The problem with DELETE, which if successful would normally return a 200 (OK) or 204 (No Content), will often return a 404 (Not Found) on subsequent calls. However, the state on the server is the same after each DELETE call, but the response is different.

GET

GET /resources (possibility of filter which returns empty list) GET /resources/{resource-id} GET /resources/{resource-name}

If GET list does not contain resources (with/without filters) return an empty list or throw exception. I prefer returning an empty list.

Example of GET list:

@GetMapping
@ApiOperation("Get information about all employees")
public Set<Employee> getAllEmployees(EmployeeFilter filter){
    LOGGER.info("Get all employees request. Employee filter {}",filter);

    return this.employeeService.getAllEmployees(filter);
}

//This will be record in the future
public class EmployeeFilter {

    private final String team;
    private final JobTitle jobTitle;

    @JsonCreator
    public EmployeeFilter(String team, JobTitle jobTitle) {
        this.team = StringUtils.normalizeSpace(team);
        this.jobTitle = jobTitle;
    }

    public Optional<String> getTeam() {
        return Optional.ofNullable(this.team);
    }

    public Optional<JobTitle> getJobTitle() {
        return Optional.ofNullable(this.jobTitle);
    }

    @Override
    public String toString() {
        return "EmployeeFilter{" +
                "team='" + this.team + '\'' +
                ", jobTitle=" + this.jobTitle +
                '}';
    }
}

Example of GET resource:

@GetMapping("{id}")
@ApiOperation("Get information about one employees")
public Employee getEmployee(@PathVariable("id") @Min(1) Long id){
    LOGGER.info("Get employee request. Employee id: {}",id);

    return this.employeeService.getEmployee(id);
}

POST

POST /employees

If employee exists, throw exception - Bad Request (400).

Questions:

  • How will respond look:
    • Return only OK;
    • Return OK and created resource's id;
    • Return OK and entire created resource (in some case this might be too costly).
  • What unique properties define resource?

Example:

@PostMapping
@ApiOperation("Add employee")
public void addEmployee(@RequestBody @Valid EmployeeRequest request){
    LOGGER.info("Add employee. Employee: {}",request);

    this.employeeService.addEmployee(request);
}

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public record EmployeeRequest(
        @NotNull Long socialSecurityNumber,
        @NotBlank @Size(max = 255) String name,
        @NotBlank @Size(max = 255) String lastName,
        @NotBlank @Size(max = 255) String team,
        @NotNull JobTitle jobTitle) {}

PUT

The PUT method requests that the state of the target resource be created or replaced with the state defined by the representation enclosed in the request message payload.

PUT is idempotent.

Questions:

  • What unique properties define resource?
  • Will you respond differently when resource was created/updated?

If the target resource does not have a current representation and the PUT successfully creates one, then the origin server MUST inform the user agent by sending a 201 (Created) response. If the target resource does have a current representation and that representation is successfully modified in accordance with the state of the enclosed representation, then the origin server MUST send either a 200 (OK) or a 204 (No Content) response to indicate successful completion of the request.

Example:

@PutMapping
@ApiOperation("Add employee or update")
public Employee putEmployee(@RequestBody @Valid EmployeeRequest request){
    LOGGER.info("Add or update employee. Request: {}",request);

    return this.employeeService.putEmployee(request);
}

PATCH

There are two standards for PATCH'ing:

JSON Patch

The following is an example JSON Patch document:

HTTP PATCH request:

PATCH /my/data HTTP/1.1
Host: example.org
Content-Length: 326
Content-Type: application/json-patch+json
If-Match: "abc123"

[
    { "op": "test", "path": "/a/b/c", "value": "foo" },
    { "op": "remove", "path": "/a/b/c" },
    { "op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ] },
    { "op": "replace", "path": "/a/b/c", "value": 42 },
    { "op": "move", "from": "/a/b/c", "path": "/a/b/d" },
    { "op": "copy", "from": "/a/b/d", "path": "/a/b/e" }
]

Not going into much detail, this kind of PATCH'ing is not intuitive and complicated. I'd recommend to stick with JSON Merge Patch.

JSON Merge Patch

For example, given the following original JSON document:

{
  "a": "b",
  "c": {
    "d": "e",
    "f": "g"
  }
}

Changing the value of "a" and removing "f" can be achieved by sending:

PATCH /target HTTP/1.1
Host: example.org
Content-Type: application/merge-patch+json
       
{
  "a":"z",
  "c": {
    "f": null
  }
}

When applied to the target resource, the value of the "a" member is replaced with "z" and "f" is removed, leaving the remaining content untouched.

To acheive this in Java, I had to create a wrapper class PatchField<T>. It solves Java's objects problem, where each object/property, if not initialized, will have default value (null for reference types, false for primitive boolean and 0 for primitive numbers). However, how can we know when null was set by consumer and when it was set during object creation.

Say, we have:

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public record EmployeeRequest(
        Long socialSecurityNumber,
        String name,
        String lastName,
        String team,
        JobTitle jobTitle) {}

If I want to nullify only name and leave the rest of Employee properties unchanged I would pass:

{
  "name": null
}

However, all objects would be initialized as nulls in this case, and it would nullify everything.

Enter PatchField<T>:

public record PatchField<T>(boolean isSet, T value) {

    public PatchField(T value) {
        this(true, value);
    }

    public static <T> PatchField<T> empty() {
        return new PatchField<>(false, null);
    }

    public void ifSet(Consumer<? super T> action) {
        if (this.isSet) {
            action.accept(this.value);
        }
    }
}

And PatchEmployeeRequest looks like so:

public class PatchEmployeeRequest {

    private PatchField<@Min(0) Long> socialSecurityNumber = PatchField.empty();
    private PatchField<String> firstName = PatchField.empty();
    private PatchField<String> lastName = PatchField.empty();
    private PatchField<String> team = PatchField.empty();
    private PatchField<JobTitle> jobTitle = PatchField.empty();

    public PatchField<Long> getSocialSecurityNumber() {
        return this.socialSecurityNumber;
    }

    public void setSocialSecurityNumber(Long socialSecurityNumber) {
        this.socialSecurityNumber = new PatchField<>(socialSecurityNumber);
    }

    //Remaining getters and Setters
    ...
}

We require to have Getters and Setters, because Jackson invokes setters only on those methods which were in the request body. In this example's case - only name. We also require to initialize each property with PatchField.empty(); to differentiate when PatchField instance set by deserializing framework and when by us.

Lastly, if you want to add jakarta validation annotations on Type parameters, like PatchField<@Min(0) Long> socialSecurityNumber, you have to create custom Extractor and register with Spring.

Extractor as per documentation:

public class PatchFieldValueExtractor implements ValueExtractor<PatchField<@ExtractedValue ?>> {

    @Override
    public void extractValues(PatchField<?> originalValue, ValueReceiver receiver) {
        receiver.value(null, originalValue.value());
    }
}

Register with Spring:

@Component
public class CustomLocalValidatorFactoryBean extends LocalValidatorFactoryBean {

    @Override
    protected void postProcessConfiguration(Configuration<?> configuration) {
        super.postProcessConfiguration(configuration);
        configuration.addValueExtractor(new PatchFieldValueExtractor());
    }
}

PATCH vs POST

Error Handling

Errors within your application should be protocol-independent. For example, if an Employee does not exist, application throws NotFoundException. Then depending on the protocol, may it be AMQP or HTTP, error should translate accordingly. For HTTP there are Spring's @RestControllerAdvice. I've provided a list of exception handlers which are utilized in my other applications. Find them under exceptionhandlers package.

Repository

Use getResultList() vs getSingleResult()

getSingleResult() throws NoResultException which is RuntimeException - if there is no result, thus you would have to wrap it into try/catch. I advise utilizing getResultList() like so:

public Optional<EmployeeEntity> getEmployee(Long id){
    var employees=em.createQuery("""
            SELECT e FROM EmployeeEntity e
            JOIN FETCH e.projects p
            JOIN FETCH e.team t
            WHERE e.id = :id""",EmployeeEntity.class)
    .setParameter("id",id)
    .getResultList();

    return Optional.ofNullable(employees.isEmpty()?null:employees.get(0));
}

Prefer JPQL versus Criteria builder.

JPQL:

public Optional<EmployeeEntity> getEmployee(Long id){
    var employees=this.em.createQuery("""
        SELECT e FROM EmployeeEntity e
        JOIN FETCH e.projects p
        JOIN FETCH e.team t
        WHERE e.id = :id""",EmployeeEntity.class)
    .setParameter("id",id)
    .getResultList();

    return Optional.ofNullable(employees.isEmpty()?null:employees.get(0));
}

VS

CriteriaBuilder:

public Optional<EmployeeEntity> getEmployee(Long id){
    var cb=this.em.getCriteriaBuilder();
    var query=cb.createQuery(EmployeeEntity.class);
    var employeeEntity=query.from(EmployeeEntity.class);
    employeeEntity.fetch("team");
    employeeEntity.fetch("projects");
    query.select(employeeEntity)
    .where(cb.equal(employeeEntity.get("id"),id));

    var employees=this.em.createQuery(query).getResultList();

    return Optional.ofNullable(employees.isEmpty()?null:employees.get(0));
}

VS

Spring Data:

@Override
@Query("""
        SELECT e FROM EmployeeEntity e
                        JOIN FETCH e.projects p
                        JOIN FETCH e.team t
                        WHERE e.id = :id""")
Optional<EmployeeEntity> findById(Long id);

or:

@Override
@EntityGraph(attributePaths = {"team", "projects"})
Optional<EmployeeEntity> findById(Long id);

We need to define JPQL query, because Spring cannot join tables optimally, unless you define fetch = FetchType.EAGER. With ManyToMany relationship, this can lead to N+1 problem. Or solution relies on using @EntityGraph. Then, fetch = FetchType.EAGER is not required.

Example of dynamic queries can be found in EmployeeRepo, EmployeeRepoCriteria and Spring's flavoured with Specification in EmployeeRepoSpringData.

TODO Remove scanning of Spring Data

Entities

equals and hashCode is implemented according to Vlad Mihalcea. More information can be found in * High-Performance Java Persistence* page 192 or in this link.

If entity can return null value, wrap getter into Optional.

Entities should not leave Service layer. Always return a view of Entity instead.

Use LocalDateTime and friends from java.time for representation of date instead of legacy Date.

Performant relationships

jpa-relationships.png

Source

For collections use Sets (@ElementCollection including) and always implement equals and hashCode according Vlad Mihalcea.

Bidirectional synchronized methods.

Even though it is adviced to use synchronized method, it's unclear why. Outstanding stackoverflow question.

Whenever a bidirectional association is formed, the application developer must make sure both sides are in-sync at all times. The addPhone() and removePhone() are utility methods that synchronize both ends whenever a child element is added or removed:

Source - Hibernate User Guide

However, we still need to have both sides in sync as otherwise, we break the Domain Model relationship consistency, and the entity state transitions are not guaranteed to work unless both sides are properly synchronized.

Source - Vlad Mihalcea Blog

Optional and nullability

Avoid nulls at all cost! Developer has to trust code otherwise you will find this pattern sprinkled Objects.requireNonNull or worse if (object != null) {} everywhere. Defend at the perimeter! (Timestamped link, watch time ~5min): Clean Code: The Next Chapter by Victor Rentea. All data which is incoming into the system from outside (REST request, RabbitMQ messages, Database entities etc.) has to follow these rules regarding nullability:

  • If property can be null, you should use Optional as a return parameter.
public class ExampleRequest {

    private final String team;

    public ExampleRequest(String team) {
        this.team = team;
    }

    public Optional<String> getTeam() {
        return Optional.ofNullable(team);
    }
}
  • If property can have default value, define it in the constructor. Another reason to use immutable types. They guarantee correct values.
@JsonCreator
public EmployeeRequest(String name,String lastName,String team,JobTitle jobTitle){
        this.name=name;
        this.lastName=lastName;
        this.team=Objects.requireNonNullElse(team,"Special Team");
        this.jobTitle=jobTitle;
        }

Here, for example, team can be null. However, if we can define a default value for that property, we should do it using requireNonNullElse or requireNonNullElseGet.

When to use Optional according to Bryan Goetz

Stackoverflow answer:

For example, you probably should never use it for something that returns an array of results, or a list of results; instead return an empty array or list. You should almost never use it as a field of something or a method parameter. I think routinely using it as a return value for getters would definitely be over-use. (!)

(Public service announcement: NEVER call Optional.get unless you can prove it will never be null; instead use one of the safe methods like orElse or ifPresent. In retrospect, we should have called get something like getOrElseThrowNoSuchElementException or something that made it far clearer that this was a highly dangerous method that undermined the whole purpose of Optional in the first place. Lesson learned. (UPDATE: Java 10 has Optional.orElseThrow(), which is semantically equivalent to get(), but whose name is more appropriate.))

Records and nullability

Intial question on OpenJDK mailing list

Suggestions from mailing list.

Either you declare the component optional:

record Person(Optional<String> name) {}

or don't declare the component Optional but burn the accessor:

record Person(String name /*may be null*/) {
    public String name() {
        throw new UnsupportedOperationException("name is optional");
    }

    public Optional<String> optionalName() {
        return Optional.ofNullable(name);
    }
}

However, there was another suggestion, which I like the most (I cannot find the source):

public record Person(Optional<String> name) {

    @JsonCreator
    public Person(String name) {
        this(Optional.ofNullable(name));
    }
}

Tests

Naming

Follow test method naming conventions methodUnderTest__[given/when]__[then] where given/when and then is optional. Example:

  • getAllEmployees__thenReturnListOfEmployees
  • getAllEmployees__whenNoEmployeesExist__thenReturnEmptyList
  • getEmployee__whenInvalidEmployeeId__thenReturn400
  • getEmployee__whenEmployeeDoesNotExists__thenReturn404

Integration Tests

Reuse Spring's Application Context as much as possible. This will speed up your tests, because it doesn't need to restart, thus in perfect conditions, you'd only need to start Spring's Application once. To achieve this:

  • Create a single annotation (see @IntegrationTest in test folder);
  • Class which each integration test extends.

Testing time

TimeMachine class.

Avoid @MockBeans

It will restart Spring's application.

Testing communication with other services

Wiremock

Contract Based Testing

End-to-End Testing

Misc

assertThat(responseFrame).hasNoNullFieldsOrProperties();

If property wrapped into Optional, it is not null, hence the assertion will pass.

Resources

https://aerokhin.com/2020/09/28/what-is-the-right-unit-in-unit-test-after-all/

https://phauer.com/2019/modern-best-practices-testing-java/

Spring

Favor construction injections vs setter/property injections. From Spring documentation:

The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null. Furthermore, constructor-injected components are always returned to the client (calling) code in a fully initialized state.

If you have multiple properties, prefer to use Type-safe Configuration Properties.

Below example, in the future, will most likely work with records.

@ConstructorBinding
@ConfigurationProperties("acme")
public class AcmeProperties {

    private final boolean enabled;
    private final int remoteAddress;
    private final String name

    public AcmeProperties(boolean enabled, int remoteAddress, String security) {
        this.enabled = enabled;
        this.remoteAddress = remoteAddress;
        this.security = security;
    }

    //Getters
}

No need to declare @Autowired on constructors if only one constructor exists.

Maven

  1. Try to have as little external dependencies as possible. Every dependency might block you in the future from migrating to newer Java version.
  2. Don't over-engineer plugins.
  3. At least in my experience it's best to have maven-compiler-plugin declared with Java versions, otherwise IntelliJ sets wrong Java for the project:
<!--plugin for intellij to get settings-->
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <release>15</release>
        <compilerArgs>--enable-preview</compilerArgs>
        <forceJavacCompilerUse>true</forceJavacCompilerUse>
        <parameters>true</parameters>
    </configuration>
</plugin>

Use UTF-8 when running surefire-plugin. There are test that include UTF-8 letters, thus if we would remove configuration they would fail.

<!-- Some tests assert on UTF-8 letters, thus without defined encoding, tests will fail.
However they fail only when running with `mvn test`, but they succeed when launched through IntelliJ UI-->
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.22.2</version>
    <configuration>
        <argLine>@{argLine} -Dfile.encoding=UTF-8</argLine>
    </configuration>
</plugin>

Don't forget to check whether surefire or jacoco supports Java version.

TODO

http://andresalmiray.com/detecting-duplicate-dependencies-in-maven/

HTTP Client

No Async/await frameworks

The performance we expect is similar to reactive frameworks. There is some added overhead, but I think it will be negligible in 99.9% of real-world use-cases.

  • Ron Pressler (Project Loom Lead)
Thread.startVirtualThread(()->{
        System.out.println("Hello, Loom!");
        });

Thread "cost"

Platform Thread Virtual Thread
> 2KB metadata 200-300B metadata
1MB Stack Pay-as-you-go stack

Context switching

Platform Thread Virtual Thread
1-10µs ~200ns

Source

Migration

If you have a common I/O operation guarded by a synchronized, replace the monitor with a ReentrantLock to let your application benefit fully from Loom’s scalability boost even before we fix pinning by monitors (or, better yet, use the higher-performance StampedLock if you can). Currently, they are working on making JDK libraries Loom-friendly.

Structured concurrency

Structured concurrency corrals thread lifetimes into code blocks. Its basic principle is this: threads that are created in some code unit must all terminate by the time we exit that code unit:

ThreadFactory vtf=Thread.builder().virtual().factory();
        try(ExecutorService e=Executors.newUnboundedExecutor(vtf)){
        e.submit(task1);
        e.submit(task2);
        } // blocks and waits

Before we exit the TWR block, the current thread will block, waiting for all tasks — and their threads — to finish. Once outside it, we are guaranteed that the tasks have terminated.

Structured Interruption

try(var e=Executors.newUnboundedExecutor(myThreadFactory)
        .withDeadline(Instant.now().plusSeconds(30))){
        e.submit(task1);
        e.submit(task2);
        }

Simple benchmarks

JFR Show threads

https://docs.oracle.com/javase/8/docs/jre/api/net/httpserver/spec/com/sun/net/httpserver/HttpServer.html

Key Takeaways

  • A virtual thread is a Thread — in code, at runtime, in the debugger and in the profiler.
  • A virtual thread is not a wrapper around an OS thread, but a Java entity.
  • Creating a virtual thread is cheap — have millions, and don’t pool them!
  • Blocking a virtual thread is cheap — be synchronous!
  • No language changes needed.
  • Pluggable schedulers offer the flexibility of asynchronous programming.

Resources:

Application.yaml

Full application.yml:

spring:
  main:
    banner-mode: off ##Turn off banner
  flyway:
    enabled: true
    baseline-on-migrate: true
    ignore-missing-migrations: true #After some time you'll want to archive migrations scripts
    locations: classpath:/db/migration/h2
    installed-by: laurynas
  datasource:
    driver-class-name: org.h2.Driver
    url: jdbc:h2:mem:citizen;DB_CLOSE_ON_EXIT=FALSE
    username: sa
    password:
    platform: h2
  jpa:
    database: h2
    show-sql: true #Use ``true`` only when you need to debug. Otherwise performance will degraded due to stoud'ing all SQL opperations 
    generate-ddl: false #Never let Hibernate/JPA generate your database schemas. Otherwise, you might find surprises if you've annotated incorrectly. See unidirectional @OneToMany. 
    hibernate:
      ddl-auto: none #Do not autogenerate SQL tables. Sometimes it might lead to suboptimal structures/types. Control you database!
    properties:
      hibernate:
        #Turn it on only when you need for analysing queries. Do not leave it on!
        generate_statistics: false
        format_sql: true
        dialect: org.hibernate.dialect.H2Dialect
    open-in-view: false #Do not allow Spring to automagically fetch data from database. See https://vladmihalcea.com/the-open-session-in-view-anti-pattern/
  h2:
    console:
      enabled: true
      path: /h2
  application:
    name: good-citizen

server:
  port: 8080
  servlet:
    context-path: /
# Custom application properties.
application:
  version: v1
  base: /api/${application.version}
  endpoints:
    employees: ${application.base}/employees
    projects: ${application.base}/projects
    teams: ${application.base}/teams


management:
  endpoints:
    enabled-by-default: false # endpoint enablement to be opt-in rather than opt-out
    web:
      exposure:
        include: health, info, prometheus, metrics #exclude everything except health, info, prometheus endpoints
  endpoint:
    health:
      enabled: true
      show-details: always
    prometheus:
      enabled: true
    metrics:
      enabled: true

Examine each step by step.

Flyway configuration

  flyway:
    enabled: true
    baseline-on-migrate: true
    ignore-missing-migrations: true #After some time you'll want to archive migrations scripts
    locations: classpath:/db/migration/h2
    installed-by: laurynas

Datasource

  datasource:
    driver-class-name: org.h2.Driver
    url: jdbc:h2:mem:citizen;DB_CLOSE_ON_EXIT=FALSE
    username: sa
    password:
    platform: h2

JPA

  jpa:
    database: h2
    show-sql: true #Use ``true`` only when you need to debug. Otherwise performance will degraded due to stoud'ing all SQL opperations 
    generate-ddl: false #Never let Hibernate/JPA generate your database schemas. Otherwise, you might find surprises if you've annotated incorrectly. See unidirectional @OneToMany. 
    hibernate:
      ddl-auto: none #Do not autogenerate SQL tables. Sometimes it might lead to suboptimal structures/types. Control you database!
    properties:
      hibernate:
        #Turn it on only when you need for analysing queries. Do not leave it on!
        generate_statistics: false
        format_sql: true
        dialect: org.hibernate.dialect.H2Dialect
    open-in-view: false #Do not allow Spring to automagically fetch data from database. See https://vladmihalcea.com/the-open-session-in-view-anti-pattern/

Management endpoints

management:
  endpoints:
    enabled-by-default: false # endpoint enablement to be opt-in rather than opt-out
    web:
      exposure:
        include: health, info, prometheus, metrics #exclude everything except health, info, prometheus endpoints
  endpoint:
    health:
      enabled: true
      show-details: always
    prometheus:
      enabled: true
    metrics:
      enabled: true

Kubernetes pod template

Database

Database migration

Database types

SQL Server

For datetime use datetime2. Mainly this stackoverflow question has all the answers:

datetime2 wins in most aspects except (old apps Compatibility)

  • larger range of values
  • better Accuracy
  • smaller storage space (if optional user-specified precision is specified)

For binary data (e.g. byte[]) save into VARBINARY(MAX):

if your pictures or document are typically below 256KB in size, storing them in a database VARBINARY column is more efficient.

Stackoverflow question and answer

Use DateTime instead of V1, V2 etc

Git

What happens when tag is equal to branch name? It will checkout to tag, instead of branch. That means, that new code has been commited to branch and CI tool would like to checkout to the head of named branch (using branch name), it will checkout to tag, and you won't have your changes deployed.

Kotlin

Kotlin and future compatibility

As far as the Java platform (AKA "the JVM") goes, Kotlin is painting itself into a corner. Kotlin was designed in 2009-10, in the Java 7 days, around the time of the Oracle acquisition, and at a time of stagnation for Java due to Sun's decline. At the time, Android was also quite similar to Java. Kotlin was a great design in those conditions. Things are very different today.

Kotlin's design goals are now contradictory. It seeks to be a low-overhead language, i.e. provide abstractions that don't add overhead over the target platform, give access to all platform capabilities, and at the same time target multiple platforms -- Java, Android, LLVM and JS -- over none of which does Kotlin exert much influence (maybe a bit on Android). Since 2010, Java has moved away from Android, to the point that there are very big gaps between the two, and they're only growing.

By both trying to target multiple platforms (with no say in their design) and trying to add little or no overhead, Kotlin will soon find itself in a bind. It's coroutines are in conflict with Java's upcoming virtual thread's design, and its inline classes will cause problems with Java's Valhalla vs. Android. Kotlin will have to choose to either give up on low-overhead abstractions, give up on some of its platforms, give up on giving access to full platform capabilities, or split into multiple languages, each targeting a different platform. For example, if Kotlin's inline types will make use of Valhalla, then they won't work on Android, at least not without much overhead.

TL;DR: A full-capability, low-overhead, multi-platform language is hard to pull off in the long run when you have low market share on all of those platforms (except Android) and little or no influence on any of them. This was viable under some accidental circumstances, but those are no longer holding.

Kotlin's other known issues

TODO

Check Prometheus endpoint with and without micrometer dependency. It might expose data which could explode cardinality.

Java SSL/TLS

Spring Security

JdbcTemplate

try{
    var framePhoto=this.jdbcTemplate.queryForObject(SELECT_ONE_SQL,Map.of("jatoId",jatoId),LcvJatoPhotoRepo::extractFrom);
    return Optional.ofNullable(framePhoto);
} catch(EmptyResultDataAccessException e){
    return Optional.empty();
}

Catch EmptyResultDataAccessException.

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